This invention relates to radiation detection devices for electromagnetic radiation, and particularly to such devices which are useful for a broadband of radio frequencies.
Described in applicant's paper "Design of Broad-Band Resistive Radiation Probes," IEEE Tr. Instr. and Meas. Nov. 1972, Vol IM-21, No. 4, pp. 416-421 and in his earlier patent U.S. Pat. No. 3,931,573 which are here incorporated by reference, is a detector for free-space radiation over a broadband of radio frequencies in the microwave spectrum. Such a detector is desirably achieved by an arrangement of resistive strips for extracting and absorbing radiant energy from the freespace field rather than by an antenna. The resistive strips keep the effects of reflection and diffraction to a minimum, and the resistive strip arrangement has sufficient transparency so as not to disturb the field being measured and detected. As explained in those prior references, the ohmic resistance of the strips is large compared to the radiation resistance over the operating frequence range. Consequently, the fractional absorption of the radiant energy by the resistive strips is substantially invariant with frequency, and an ultra broadband radiation detecting device is thereby produced. Thus, unlike an antenna, which uses metallic surfaces of negligible resistance, the resistive strip arrangement does not disturb the near field being measured. Applicant's aforementioned paper and patent describe a specific embodiment using thermocouples for converting the absorbed r-f emergy to a d-c measurement signal.
Other radiation detectors use antennas (i.e., transducers having conventional highly conductive surfaces). These detectors tend to produce disturbance in the field being measured due to the conductive materials, and to be narrow band by reason of the frequency sensitivity of the antenna. However, by using a "short" dipole antenna (i.e., a small fraction of a wavelength at the high frequency end of the band), and with a high reactance relative to the free space impedance, somewhat broader bands have been achieved; see "New Near-Zone Electric-Field-Strength Meter" by F. M. Greene, NBS J. of Research--C. Eng. and Instr., Vol. 71C, No. 1, Jan.-Mar. 1967, pp. 51-57; "Near Field Instrumentation," report of A. W. Rudge, et al, July 1970, Bureau of Radiological Health of U.S. Dept. H.E.W.; and U.S. Pat. No. 3,750,017 of Bowman, et al. These detectors use diodes for converting the r-f currents induced in the antennas to a d-c measurement signal, which have the advantage of higher sensitivity. Practical constraints may limit such detectors to frequencies up to a few gigaHerz.